AP Syllabus focus:
‘In meiosis II, spindle fibers attach to sister chromatids, which align individually along the metaphase plate.’
Meiosis II is a second division that follows meiosis I.
Overview diagram of meiosis highlighting that meiosis II begins with haploid cells whose chromosomes are still duplicated (sister chromatids joined at the centromere). The figure also depicts metaphase II alignment at the cell equator followed by anaphase II separation of sister chromatids into four haploid products. Source
It separates sister chromatids in each haploid cell, using the spindle apparatus to ensure accurate chromosome movement and alignment before chromatid separation.
Big picture: what meiosis II starts with
After meiosis I and cytokinesis, cells enter meiosis II with key features that shape prophase II and metaphase II:
Each cell is haploid (one chromosome from each homologous pair).
Each chromosome is still duplicated, consisting of two sister chromatids joined at a centromere.
There is no DNA replication between meiosis I and meiosis II (often described as interkinesis).
Prophase II: building a new spindle in each haploid cell
Core events
Prophase II reorganizes each haploid cell so chromosomes can be positioned for separation:
Chromosomes condense (if they partially decondensed after meiosis I), becoming more visible and compact.
The nuclear envelope (if present) breaks down, allowing spindle microtubules to access chromosomes.
Centrosomes (or other microtubule-organizing centers) move toward opposite poles of the cell.
A new spindle apparatus forms from microtubules and associated proteins.
What is being “prepared” in prophase II
The critical setup is for microtubules to capture chromatids:
Microtubules grow and shrink dynamically, increasing the chance of contacting chromosomes.
Chromosome structure positions attachment sites so that, in the next stage, spindle fibres can attach to each sister chromatid.
Key vocabulary for this stage
DEFINITION
Kinetochore: A protein complex assembled on the centromere of each chromatid that serves as the attachment site for spindle microtubules and helps move chromosomes.
Kinetochore formation and accessibility are essential because meiosis II depends on attaching spindle fibres to each chromatid rather than to homologous chromosome pairs.
Molecular-scale illustration of a kinetochore attached to a microtubule, emphasizing the kinetochore as the physical interface that couples chromosome movement to spindle microtubules. This helps connect the definition of “kinetochore” to the real, protein-based machinery required for spindle fibers to capture and move sister chromatids. Source
Metaphase II: individual chromosomes line up
The syllabus focus in action
In metaphase II, the defining events match the syllabus statement exactly:
Spindle fibers attach to sister chromatids via their kinetochores.
Chromosomes align individually along the metaphase plate, not as homologous pairs.
How attachment and alignment work
Correct metaphase II alignment depends on a specific orientation:
Each chromosome is positioned so that the kinetochore of one sister chromatid attaches to microtubules from one pole.
The kinetochore of the other sister chromatid attaches to microtubules from the opposite pole.
This creates tension across the centromere, helping stabilize proper attachments.
The metaphase plate is a functional plane (not a structure) where chromosomes are most likely to be evenly distributed before chromatid separation.
Why metaphase II differs from metaphase I
Metaphase II is often confused with metaphase I, but the alignment unit is different:
Metaphase I: homologous chromosome pairs align together.
Metaphase II: single chromosomes align, with sister chromatids facing opposite poles.
This distinction matters because meiosis II is mechanistically similar to mitosis in how chromatids are handled, but it occurs in haploid cells whose chromatids may not be genetically identical.
Accuracy checkpoints students should know
Even at the AP level, it is useful to connect structure to function:
Review-figure schematic showing how kinetochore–microtubule attachment stability is regulated, including a plot that relates microtubule-binding affinity to kinetochore phosphorylation state. It provides a mechanistic backdrop for why improper attachments are destabilized, while correct bi-oriented attachments that generate tension are selectively stabilized before chromatid separation. Source
Stable kinetochore–microtubule attachments promote correct alignment.
Improper attachment can prevent stable alignment at the metaphase plate, reducing the likelihood of equal chromatid distribution when separation begins.
FAQ
Kinetochores are dynamic protein assemblies. Their composition and microtubule attachments are remodelled between divisions.
This rebuilding helps switch from meiosis I attachment patterns to meiosis II bi-orientation.
Cells do not enter an S phase during interkinesis. Regulatory cyclins/CDKs maintain progression without relicensing DNA replication origins.
This ensures chromatids present in metaphase II are the same ones produced before meiosis I.
Correct attachments are stabilised when tension is generated across the centromere.
Incorrect attachments tend to be less stable and are more likely to detach and retry until proper bi-orientation occurs.
Not necessarily. If recombination occurred earlier, sister chromatids can carry different allele combinations.
They are still “sisters” because they remain joined at the centromere until separation begins.
Because homologous pairs were separated in meiosis I. Each remaining chromosome behaves as a single unit at the metaphase plate.
This contrasts with metaphase I, where paired homologues align together.
Practice Questions
Describe what happens to chromosomes during metaphase II. (2 marks)
Chromosomes (each with two sister chromatids) align individually on the metaphase plate (1).
Spindle fibres attach to kinetochores on sister chromatids from opposite poles (1).
Explain how events in prophase II enable the alignment seen in metaphase II. (5 marks)
Chromosomes condense, making them easier to move without tangling (1).
Nuclear envelope breaks down, allowing spindle access to chromosomes/kinetochores (1).
Spindle forms from microtubules organised at opposite poles (1).
Spindle fibres attach to kinetochores on sister chromatids (1).
Opposite-pole attachment allows chromosomes to align individually at the metaphase plate (1).
